Improve your self-discipline. In order to succeed academically online, you need to have a drive to excel in your studies. You'll have to motivate yourself to log on and complete your course requirements.
Work on your writing and grammar. Since you're communicating through forums and email, you want to work on correcting bad grammar and writing proficiently.
Schedule set times to complete assignments. Make weekly commitments to spend time on your online studies. Try not to deviate from the schedule.
Contribute as much as possible. If you want to succeed and earn a high grade, you can't just fade into the background. Post to online class forums and communicate with your professor often via email.
Get help when needed. If you're having trouble meeting the demands of the online class, try to eliminate unnecessary commitments from your schedule. Also, recruit the support of your parents and friends while taking online classes.